There has been substantial interest in the preparation and characterisation of compound semiconductors consisting of particles with dimensions in the order of 2-100 nm. Often referred to as quantum dots and/or nanocrystals, this is mainly because of their optical, electronic and chemical properties. These studies have occurred mainly due to their size-tuneable electronic, optical and chemical properties and the need for the further miniaturization of both optical and electronic devices that now range from commercial applications as diverse as biological labelling, solar cells, catalysis, biological imaging, light-emitting diodes amongst many new and emerging applications.
Although some earlier examples appear in the literature, recently methods have been developed from reproducible “bottom up” techniques, whereby particles are prepared atom-by-atom, i.e., from molecules to clusters to particles using “wet” chemical procedures. Rather from “top down” techniques involving the milling of solids to finer and finer powders.
To-date the most studied and prepared of semiconductor materials have been the chalcogenides II-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSe due to its tuneability over the visible region of the spectrum. As mentioned semiconductor nanoparticles are of academic and commercial interest due to their differing and unique properties from those of the same material, but in the macro crystalline bulk form. Two fundamental factors, both related to the size of the individual nanoparticle, are responsible for their unique properties. The first is the large surface to volume ratio; as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor is that, with semiconductor nanoparticles, there is a change in the electronic properties of the material with size, moreover, the band gap gradually becoming larger because of quantum confinement effects as the size of the particles decreases. This effect is a consequence of the confinement of an ‘electron in a box’ giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the “electron and hole”, produced by the absorption of electromagnetic radiation, a photon, with energy greater then the first excitonic transition, are closer together than in the corresponding macrocrystalline material, so that the Coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission, which is dependent upon the particle size and composition. Thus, quantum dots have higher kinetic energy than the corresponding macrocrystalline material and consequently the first excitonic transition (band gap) increases in energy with decreasing particle diameter.
Single core nanoparticles, which consist of a single semiconductor material along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dagling bonds situated on the nanoparticle surface which lead to non-radiative electron-hole recombinations. One method to eliminate defects and dangling bonds is to grow a second material, having a wider band-gap and small lattice mismatch with the core material, epitaxially on the surface of the core particle, (e.g. another II-VI material) to produce a “core-shell particle”. Core-shell particles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centres. One example is ZnS grown on the surface of CdSe cores. The shell is generally a material with a wider bandgap then the core material and with little lattice mismatch to that of the core material, so that the interface between the two materials has as little lattice strain as possible. Excessive strain can further result in defects and non-radiative electron-hole recombination resulting in low quantum efficiencies.
The coordination about the final inorganic surface atoms in any core, core-shell or core-multi shell nanoparticles is incomplete, with highly reactive “dangling bonds” on the surface, which can lead to particle agglomeration. This problem is overcome by passivating (capping) the “bare” surface atoms with protecting organic groups. The capping or passivating of particles not only prevents particle agglomeration from occurring, it also protects the particle from its surrounding chemical environment, along with providing electronic stabilization (passivation) to the particles in the case of core material. The capping agent usually takes the form of a Lewis base compound covalently bound to surface metal atoms of the outer most inorganic layer of the particle, but more recently, so as to incorporate the particle into a composite, an organic system or biological system can take the form of, an organic polymer forming a sheaf around the particle with chemical functional groups for further chemical synthesis, or an organic group bonded directly to the surface of the particle with chemical functional groups for further chemical synthesis.
Many synthetic methods for the preparation of semiconductor nanoparticles have been reported, early routes applied conventional colloidal aqueous chemistry, with more recent methods involving the kinetically controlled precipitation of nanocrystallites, using organometallic compounds.
Over the past six years the important issues have concerned the synthesis of high quality semiconductor nanoparticles in terms of uniform shape, size distribution and quantum efficiencies. This has lead to a number of methods that can routinely produce semiconductor nanoparticles, with monodispersity of <5% with quantum yields >50%. Most of these methods are based on the original “nucleation and growth” method described by Murray, Norris and Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706, incorporated by reference in its entirety, but use other precursors than the organometallic ones used. Murray et al originally used organometallic solutions of metal-alkyls (R2M) M=Cd, Zn, Te; R=Me, Et and tri-n-octylphosphine sulfide/selenide (TOPS/Se) dissolved in tri-n-octylphosphine (TOP). These precursor solutions are injected into hot tri-n-octylphosphine oxide (TOPO) in the temperature range 120-400° C. depending on the material being produced. This produces TOPO coated/capped semiconductor nanoparticles of II-VI material. The size of the particles is controlled by the temperature, concentration of precursor used and length of time at which the synthesis is undertaken, with larger particles being obtained at higher temperatures, higher precursor concentrations and prolonged reaction times. This organometallic route has advantages over other synthetic methods, including near monodispersity <5% and high particle cystallinity. As mentioned, many variations of this method have now appeared in the literature which routinely give high quality core and core-shell nanoparticles with monodispesity of <5% and quantum yield >50% (for core-shell particles of as-prepared solutions), with many methods displaying a high degree of size and shape control.
Recently attention has focused on the use of “greener” precursors that are less exotic and less expensive but not necessarily more environmentally friendly. Some of these new precursors include the oxides, CdO; carbonates MCO3 M=Cd, Zn; acetates M(CH3CO2) M=Cd, Zn and acetylacetanates [CH3COOCH═C(C—)CH3]2 M=Cd, Zn; amongst others.
The use of the term “greener” precursors in semiconductor particle synthesis has generally taken on the meaning of cheaper, readily available and easier to handle precursor starting materials, than the originally used organometallics which are volatile and air and moisture sensitive, and does not necessarily mean that “greener precursors” are any more environmentally friendly.
Single-source precursors have also proved useful in the synthesis of semiconductor nanoparticle materials of II-VI, as well as other compound semiconductor nanoparticles. Bis(dialkyldithio-/diseleno-carbamato)cadmium(II)/zinc(II) compounds, M(E2CNR2)2 (M=Zn or Cd, E=S or Se and R=alkyl), have used a similar ‘one-pot’ synthetic procedure, which involved dissolving the precursor in tri-n-octylphosphine (TOP) followed by rapid injection into hot tri-n-octylphosphine oxide/tri-n-octylphosphine (TOPO/TOP) above 200° C.
Due to their increased covalent nature III-V and IV-VI highly crystalline semiconductor nanoparticles are more difficult to prepare and much longer annealing time are usually required. However, there are now many reports of II-VI and IV-VI materials being prepared by a similar procedure GaN, GaP, GaAs, InP, InAs and for PbS and PbSe.
For all of the above methods, rapid particle nucleation followed by slow particle growth is essential for a narrow particle size distribution. All these synthetic methods are based on the original organometallic “nucleation and growth” method by Murray et al which involves the rapid injection of the precursors into a hot solution of a Lewis base coordinating solvent (capping agent) which may also contain one of the precursors. The addition of the cooler solution subsequently lowers the reaction temperature and assist particle growth but inhibits further nucleation. The temperature is then maintained for a period of time, with the size of the resulting particles depending on reaction time, temperature and ratio of capping agent to precursor used. The resulting solution is cooled followed by the addition of an excess of a polar solvent (methanol or ethanol or sometimes acetone) to produce a precipitate of the particles that can be isolated by filtration or centrifugation.
Fundamentally, these preparations rely on the principle of particle nucleation followed by growth. Moreover, to have a monodispersed ensemble of nanoparticles there is preferably proper separation of nanoparticles nucleation from nanoparticle growth with the later occurring at a lower temperature from the former. This is achieved by rapid injection of one or both precursors into a hot coordinating solvent (containing the other precursor if otherwise not present) which initiates particles nucleation, however, the sudden addition of the cooler solution upon injection subsequently lowers the reaction temperature (the volume of solution added is about ⅓ of the total solution) and inhibits further nucleation maintaining a narrow nanoparticle size distribution. Particle growth being a surface catalysis process or via Ostwald ripening, depending on the amount of precursor available to the growing particles continues at the lower temperature and thus nucleation and growth are separated. This method works well for small scale synthesis where one solution can be added rapidly to another while keeping an homogenous temperature throughout the reaction. However, on larger preparative scale whereby large volumes of solution are required to be rapidly injected into one another a temperature differential can occur within the reaction which can subsequently lead to a large particle size distribution.
Preparation from single-source molecular clusters, Cooney and co-workers used the cluster [S4Cd10(SPh)16] [Me3NH]4 to produce nanoparticles of CdS via the oxidation of surface-capping SPh− ligands by iodine. This route followed the fragmentation of the majority of clusters into ions which were consumed by the remaining [S4Cd10(SPh)16]4− clusters which subsequently grow into nanoparticles of CdS. See LØver, T.; Bowmaker, G. A.; Seakins, J. M.; Cooney, R. P.; Henderson, W. J. Mater. Chem., 1997, 7(4), 647, incorporated herein in its entirety. Strouse and co-workers used a similar synthetic approach but employed thermolysis (lyothermal-elevation of temperature) rather than a chemical agent to initiate particle growth. See Cumberland, S. L.; Hanif, K. M.; Javier, A., Khitov, K., A.; Strouse, G. F., Woessner, S. M., Yun, C. S., Chem. Mater. 2002, 14, 1576. Moreover, the single-source precursors [M10Se4(SPh)16][X]4X═Li+ or (CH3)3NH+, M=Cd or Zn were thermolysised whereby fragmentation of some clusters occurs followed by growth of other from the scavenging of the free M and Se ions or simply from clusters aggregating to form larger clusters and then small nanoparticles which subsequently continue to grow into larger particles.